You could add an ExecStartPre= line to dump1090-fa.service to run whatever you need immediately before dump1090-fa starts
You can either edit /lib/systemd/system/dump1090-fa.service directly - but be aware that your changes will be lost on package upgrade - or create a new file /etc/systemd/system/dump1090-fa.service.d/bias-t.conf that has only the ExecStartPre= line and it will be merged with the service file in /lib